Dumbledore should also be in the story. His vast knowledge and foresight can either be a help or a hindrance to Sirius and Harry. He might be the one to discover their time - traveling secret and then either guide them or try to stop them depending on how he perceives the changes they are making to the timeline.
Obviously, Sirius and Harry are the main ones. But including Remus Lupin can add depth. Since he was part of the Marauders and was close to Sirius, his presence can create some interesting dynamics. He can also offer his wisdom and werewolf - related knowledge to help in their time - traveling adventures.